Preseeding Full Disk Encryption

Usually I try to write articles that are not aimed at a particular distribution. Although I may give examples assuming a Debian-based distribution, whenever possible, I try to make my instructions applicable to everyone. This is not going to be one of those articles. Here, I document a process I went through recently with Debian preseeding (a method of automating a Debian install, like kickstart on Red Hat-based systems) that I found much more difficult than it needed to be, mostly because documentation was so sparse. In fact, I really found only two solid examples to work from in my research, one of which referred to the other.

In this article, I describe how to preseed full-disk encryption in a Debian install. This problem came up as I was trying to create a fully automated "OEM" install for a laptop. The goal was to have an automated boot mode that would guide users through their OS install and use full-disk encryption by default, but would make the process as simple as possible for users. Normally, unless you are going to encrypt the entire disk as one big partition, the Debian installer makes you jump through a few hoops to set up disk encryption during an install.

In my case, I couldn't just use the full disk, because I needed to carve off a small section of the disk as a rescue partition to store the OEM install image itself. My end goal was to make it so users just had to enter their passphrase, and it would set up an unencrypted /boot and rescue disk partition and an encrypted / and swap. As an additional challenge, I also wanted to skip the time-consuming disk-erasing process that typically happens when you enable disk encryption with Debian, since the disk was going to be blank to start with anyway.

Unfortunately, although there is a lot of documentation on how to automate ordinary partitioning and LVM with preseeding (I actually wrote a whole section on the topic myself in one of my books), I had a hard time finding much documentation on how to add encryption to the mix. After a lot of research, I finally found two posts (and as I mentioned, one of them referenced the other) that described the magic incantation that would enable this. Unfortunately, the only supported mode for encrypted disks in Debian preseed requires the use of LVM (something I confirmed later when I read the source code responsible for this part of the install). That's not the end of the world, but it would have been simpler in my mind if it didn't have that requirement.

Since you need a basic unencrypted /boot partition to load a kernel and prompt the user for a passphrase, I had to account for both and preserve a small 2GB rescue disk partition that already was present on the disk. After that, the remaining / and swap partitions were encrypted. Here is the partition section of the preseed config:


d-i partman-auto/method string crypto
d-i partman-lvm/device_remove_lvm boolean true
d-i partman-lvm/confirm boolean true
d-i partman-auto-lvm/guided_size string max
d-i partman-auto-lvm/new_vg_name string crypt
d-i partman-auto/disk string /dev/sda
d-i partman-auto/choose_recipe select root-encrypted
d-i partman-auto/expert_recipe string                         \
      root-encrypted ::                                       \
              500 500 500 ext3                                \
                      $primary{ } $bootable{ }                \
                      method{ format } format{ }              \
                      use_filesystem{ } filesystem{ ext4 }    \
                      mountpoint{ /boot }                     \
              .                                               \
              2000 2000 2000 linux-swap                       \
                      $lvmok{ } lv_name{ swap }               \
                      in_vg { crypt }                         \
                      $primary{ }                             \
                      method{ swap } format{ }                \
              .                                               \
              500 10000 1000000000 ext4                       \
                      $lvmok{ } lv_name{ root }               \
                      in_vg { crypt }                         \
                      $primary{ }                             \
                      method{ format } format{ }              \
                      use_filesystem{ } filesystem{ ext4 }    \
                      mountpoint{ / }                         \
              .                                               \
              2000 2000 2000 ext4                             \
                      $primary{ }                             \
                      method{ keep }                          \
                      use_filesystem{ } filesystem{ ext4 }    \
                      label{ rescuedisk }                     \
              .

d-i partman-md/device_remove_md boolean true
d-i partman-basicfilesystems/no_mount_point boolean false
d-i partman-partitioning/confirm_write_new_label boolean true
d-i partman/choose_partition select finish
d-i partman/confirm boolean true
d-i partman/confirm_nooverwrite boolean true



If you've never worked with preseeding, this entire section of code probably looks incredibly foreign. As preseeding in general is documented well in a number of other places, I'm not going to bother breaking down every setting here. Instead, let me highlight the settings that matter for disk encryption. The most important one tells partman (the preseed partition manager) to use encryption:


d-i partman-auto/method string crypto

Next, because preseeded encrypted partitions need to use LVM, I must add LVM-specific preseed settings:


d-i partman-lvm/device_remove_lvm boolean true
d-i partman-lvm/confirm boolean true
d-i partman-auto-lvm/guided_size string max
d-i partman-auto-lvm/new_vg_name string crypt

In the last of these settings, I told partman to create a new LVM volume group named crypt that I will use to store my encrypted partitions. Further down when I define my swap and root partitions, you can see where I defined the logical volumes by name and set what volume group they are in:


2000 2000 2000 linux-swap                       \
        $lvmok{ } lv_name{ swap }               \
        in_vg { crypt }                         \
. . .
500 10000 1000000000 ext4                       \
        $lvmok{ } lv_name{ root }               \
        in_vg { crypt }                         \

Once these settings were in place, I was able to preseed an install and have disk encryption be almost fully automated, except that the installer prompted me for a passphrase, which I wanted.

The only missing piece to this automation was that the installer started overwriting the existing disk with random information. Now, there are good reasons why you may want to do this before setting up disk encryption, but in this case, the disk was blank beforehand, and I didn't want to wait the many hours it might take. Try as I might, no options to preseed this feature away seemed to work. After poring through the partman code to find the magic option, I finally resorted to patching the partman-crypto script on the fly in the middle of the install so that it skipped the erase process:


d-i partman/early_command \
       string sed -i.bak 's/-f $id\/skip_erase/-d $id/g'
/lib/partman/lib/crypto-base.sh

This is an ugly hack indeed, but it was the only way I was able to find that worked. With that in place, I was able have an automated partitioning recipe with full-disk encryption that skipped the disk-erasing section. My hope is that the next time other people need to do this and do a search on-line, they at least can find my article and the two other examples and won't have to burn so much time.

Kyle Rankin is a Tech Editor and columnist at Linux Journal and the Chief Security Officer at Purism. He is the author of Linux Hardening in Hostile Networks, DevOps Troubleshooting, The Official Ubuntu Server Book, Knoppix Hacks, Knoppix Pocket Reference, Linux Multimedia Hacks and Ubuntu Hacks, and also a contributor to a number of other O'Reilly books. Rankin speaks frequently on security and open-source software including at BsidesLV, O'Reilly Security Conference, OSCON, SCALE, CactusCon, Linux World Expo and Penguicon. You can follow him at @kylerankin.

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